Wildlife and Bird Photography lens

I'm currently using D7000 with 55-200mm. The zoom is not enough for birding. Thus i'm looking for super telephoto. My budget is limited (around $800). So i found Sigma 150-500 OS is the best option available.

Whats your opinion about 150-500mm F5-6.3 APO DG OS HSM ?

Is there any other good super telephoto lenses available for that price range ?